1. Field of the Invention
The present invention relates to the production of ceramic articles based on silicon nitride and, more especially, to the production of such ceramics from organosilicon compounds of the polycarbosilane type.
2. Description of the Prior Art
The production of ceramic articles by thermal degradation of organosilicon polymers in a controlled atmosphere is known to this art; numerous literature articles and patents exist on the subject.
One advantage of this "polymer" technique resides, in particular, in the possibilities for shaping this type of product, notably into ceramic fibers, after pyrolysis thereof.
According to the conventional technique, the polymer precursors of polycarbosilane type (possibly after the melting thereof, if originally they are in the solid state) are extruded in the form of continuous fibers (or filaments), which subsequently are treated, in particular to improve their high temperature and/or mechanical strength, and then pyrolyzed in a suitable atmosphere to produce the desired ceramic fiber.
The pretreatment of the fibers prior to pyrolysis, typically designated a hardening, infusibility or crosslinking treatment, is an essential stage in any process for the preparation of ceramic fibers.
At the present time, the hardening of polycarbosilane fibers entails either physical techniques (electronic, UV-irradiation, etc.) or chemical techniques.
The aforementioned physical techniques have the major disadvantage of being awkward and expensive. Therefore, the only economical technique utilized on an industrial scale is chemical hardening by means of an oxygen treatment.
It too is known to this art:
(a) that the pyrolysis of a polycarbosilane pretreated physically or with oxygen, under an inert atmosphere or in vacuum, provides a ceramic material based on silicon carbide;
(b) that the pyrolysis under ammonia of a polycarbosilane pretreated with oxygen in all cases provides ceramic articles containing silicon oxynitride values (as a result of the introduction of oxygen during the pretreatment); and, finally,
(c) that the pyrolysis under ammonia of a physically pretreated polycarbosilane provides, depending upon the temperature used, either silicon carbonitride or silicon nitride (partial or total substitution of carbon atoms by nitrogen atoms).
Therefore, serious need continues to exist in this art for a process for the preparation of silicon nitride from polycarbosilanes which avoids the disadvantages resulting from the necessary utilization of the aforementioned physical pretreatments.